Methodology

The parts count method is a technique for developing
an estimate or prediction of the average life, the Mean Time Between
Failures
(MTBF), of an assembly. It is a prediction process whereby a
numerical
estimate is made of the ability, with respect to failure, of a design
to
perform its intended function. Once the failure rate is
determined,
MTBF is easily calculated as the inverse of the failure rate, as
follows:

MTBF =
1
FR1 + FR2 + FR3 + ...........FRn

where FR is the failure rate of each component
of the system up to n, all components

The general procedure for determining a board
level (or system level) failure rate is to sum individual failure rates
for each component. For MIL-HDBK-217, the summation is then added
to a failure rate for the circuit board, which includes the affect of
solder
joints. Component failure rates are provided by MIL-HDBK-217, "Military
Handbook, Reliability Prediction of Electronic Equipment", as standard
part failure rate models or directly from the manufacturers.

The failure rates presented apply to equipment
under normal operating conditions, i.e., with power on and performing
its
intended function in its intended environment. Consideration is
given
to various environments, component quality, and thermal aspects.

The Equations
A sample calculation for integrated circuits
taken from MIL-HDBK-217 is as follows:

Failure Rate = (C1 * PiT + C2 * PiE) * PiQ * PiL

Each factor in this equation is dependent upon
a certain part parameter. The end result of this equation is the
failure
rate of the integrated circuit.

Failure Rate, MTBF, and FITs

For this discussion, we will assume that the resulting
failure rate is shown in failures per million hours. This is simply the
number of failures that you would expect to have in a million hours of
operation of your equipment. Failure rates for many basic devices are
well
below 1 failure per million hours, so these values may seem
insignificant.
But if you have hundreds of parts in your design and have a thousand
systems
operating in the field, you can see that the failure rates will quickly
add up. MTBF, or Mean Time Between Failures, is the inverse of the
failure
rate and is the average time between failures. It is calculated from
the
failure rate as follows:

MTBF = 1,000,000/Failure Rate

You can choose the units in which the failure
rate is shown. Another common unit used, besides failures/million
hours,
is failures per billion hours which is also known as FITs (Failures In
Time).